The present invention relates to a pneumatic tire with a tread reinforcing belt, more particularly to an improved belt being suitable for radial tires for passenger cars.
Hitherto, belt cords having a 1xc3x972, 1xc3x973, 1xc3x974 or 2+2 structure made up of two or three or four steel filaments have been widely used in a tread reinforcing belt of a passenger car radial tire because to decrease the number of the filaments by increasing the diameters of steel filaments are favorable for the cost and production efficiency of the cord. However, such a cord structure is not always favorable for ride comfort which became especially important to the recent high-class passenger cars because the bending rigidity of the belt is relatively high due to the relatively large diameter of the steel filaments. If adhesion failure occurs between the steel cords and surround topping rubber and tread rubber due to large difference in stiffness therebetween, a decrease in the cord strength due to corrosion and a resultant decrease in the tread durability tend to be caused. On the other hand, for the steel cords made up of three or four steel filaments, in order to improve rubber penetration into a closed space among the filaments, there has been proposed such a technique that the closed space is opened by waving one or more of the filaments. In such a cord, therefore, the cord diameter inevitably increases and the thickness of the belt ply increases. This is not preferable for the tire weight and belt rigidity.
It is an object of the present invention to provide a pneumatic tire in which ride comfort is improved, and adhesion failure between the belt cords and surrounding rubber can be effectively prevented.
According to the present invention, a pneumatic tire comprises
a belt disposed in a tread portion, the belt made of cords each made up of six to twelve metallic filaments whose diameter is not less than 0.15 mm but less than 0.25 mm,
the six to twelve metallic filaments being grouped into a plurality of bunches, each bunch including two to four filaments being twisted together into the bunch at a first twist pitch,
the bunches being twisted together into the cord at a final twist pitch, the final twist pitch being in a range of from 10 to 40 mm, and the first twist pitch being more than the final twist pitch, and
each of the bunch including at least one waved filament which is two-dimensionally waved at wave pitches in a range of from 5 to 30 times the diameter of the filament and at a wave height in a range of from 0.2 to 3.0 times the diameter of the filament.